Medical Plastics: Turning to the humble gecko

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NEW ORLEANS -- Polymers for use in the medical industry are most commonly processed in all the usual ways: injection molding, extrusion, blow molding.

But beyond the realms of standard medical devices, some curious and alien techniques come into effect.

These are being used in order to create intricate and detailed surfaces that have very specific medical effects.

For example, scientists in the United States have created microscopic polyurethane structures that mimic the way that a gecko's foot sticks to a surface.

Conventional adhesives are "pressure sensitive," and are widely effective, but not in all environments. They struggle in a vacuum, for example, or in aqueous environments – such as inside the human body.

For this reason, scientists are looking to develop adhesives that work using new mechanisms, such as by mimicking the gecko's foot – which leaves no sticky residue behind.

The pads on a gecko's feet are composed of millions of tiny "hairs": these make such intimate contact with the surface, that forces of molecular attraction – called Van der Waals forces – come into effect.

"Usually, you can never get close enough contact with a surface for this to happen," says Noshir Pesika, assistant professor in the chemical and biomolecular engineering department at Tulane University.

He has used photolithography – a technique normally used in the production of microchips – to create the special adhesive surfaces that mimic the gecko's foot. He has produced tiny polyurethane structures with microscopic features on the surface, which stick through "attraction" only.

The main body of the structure is a cylinder. On top of this, a series of tiny fibers protrude from the surface. These measure around 20 x 20 x 7 microns. It is these tiny fibers, or pads, that make intimate contact with a surface and stick to it via molecular attraction.

For comparison, the equivalent structure on a gecko's foot is on the nano-scale, and around a thousand times smaller.

However, these tiny structures on the gecko's foot are relatively stiff. Pesika's structures can be made more pliable, which improves surface contact. He estimates that his structures are around a hundred times weaker than a real gecko's foot. But he adds that a 1-foot-by-1-foot pad of his material could support the weight of a human.

"So far, we've made structures that are about 1.5 inches square," he said.

The samples were made at the Naval Research Laboratory in Washington.

A key attribute of the gecko's foot is its anisotropy: this means that it sticks firmly when sheared in one direction, but weakly in the other. It is this quality that allows the gecko to run quickly across the ceiling.

"The gecko's foot sticks strongly but comes off easily," he says.

Pesika has reproduced this effect by mimicking the structure of the gecko's foot, tilting the small fibers at an angle to create the anisotropy.

He points to a number of potential markets: space travel, where the vacuum of space can destroy conventional products; and in aqueous environments – which could include the inside of the human body.

"It could be used to make special sutures, which surgeons could 'stick and replace' in order to put them in the correct position," he said.

Pesika envisages an initial product in the form of a tape, but where the sticky surface is protected until it is needed. But launching a commercial product is likely to be some way off – and many challenges remain.

"To commercialize this, it would have to be made in a continuous process, rather than the batch process we're currently using," he said.

At the same time, researchers at the University of Akron are looking into the gripping power of the gecko's foot in wet conditions – in the hope that the results can be used to develop new types of bandage and other surgical products.

The gecko's foot naturally repels water, allowing it to stick to wet surfaces. The researchers, led by Alyssa Stark, have tested gecko toe hair adhesion in a series of scenarios: dry toe pads on dry, misted and wet surfaces; and soaked toe pads on dry, misted and wet glass.

The soaked toe pads showed low to no adhesion depending on the amount of wetness. Likewise, dry toe pads lost their adhesive grip increasingly with the amount of water applied to the surface upon which they were pulled.

For the experiments, geckos were pulled on a glass surface by a small harness placed around their midsections.

"We are gathering clues about how geckos interact with wet surfaces and this gives us ideas of how to design adhesives that work under water," said Ali Dhinojwala, chair of the department of polymer science.

"Nature gives us a certain set of rules that point us in the right direction. They help us understand limitations and how to manipulate materials."

Dhinojwala and colleagues have already developed a dry synthetic adhesive, comprised of carbon nanotubes, that outperforms nature's variety.

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